Draft sills have been used to receive and house coupler systems for coupling one railroad car to another. Buff and draft forces are generally transferred between the draft sill structure, the car truck and the center sill of the car. More particularly, the draft sill structure typically receives a coupler and a yoke or other coupler mounting structure such as a draw bar and a cushioning or shock-absorbing assembly.
Continued attempts have been made to decrease the weight of railroad cars to allow for reduced energy consumption and more efficient rail transport. It has been desirable to produce railroad freight car components that are relatively lightweight and that can accommodate new car designs. In some instances, attempts have been made to reduce the weight of the draft sills themselves. Attempts have been made to produce lighter weight fabricated draft sills instead of using cast draft sill structures.
Typical fabricated sills have front and rear stops, ribs and center plate that are welded into place between side walls in the draft sill. The fabricated structure has been welded to a shear plate on the underside of the railroad car. Fabricated draft sills have typically had the advantage of being lighter in weight than cast draft sills, but have also had the disadvantage of having a generally shorter fatigue life compared to cast draft sills, due in part to increased stresses at the numerous welds in the fabricated sills.
The Association of American Railroads (AAR) has set requirements for the strength of draft sill structures. For example, the rear stops must be capable of withstanding a 1000 Klbs static buff load without failure, and future AAR requirements will require that the front stops be capable of withstanding a 900 Klbs static draft force. Given these strength requirements, it has been problematic to provide a lightweight cast draft sill that meets AAR strength requirements.